1. Field of the Invention
The present invention relates to a fixing unit for use with an image forming apparatus, such as a copying machine, a printer, or a facsimile device. More particularly, the invention relates to a fixing unit of the roll-belt type having a roll and an endless belt, which can meet the demands of high fixing speed and size reduction, and to an endless belt used for the fixing unit.
2. Description of Prior Art
There has been known a heating/pressure roll fixing unit (referred frequently to as a roll-roll fixing unit) as shown in FIG. 3. In the this type of the fixing unit, a pair of rolls having heating function, a fixing roll 1 and a pressure roll 2, are arranged to be in press contact with each other, as shown. A recording sheet 3 carrying thereon a toner image not yet fixed is put into the nip area between the paired rolls 1 and 2. When the recording sheet 3 passes through the nip, the toner image 4 is heated under pressure, so that the toner image is fixed onto the recording sheet 3.
The fixing roll 1 of the fixing unit is composed of a hollowed roll 1a made of metal of high thermal conductivity, such as aluminum, an elastic layer 1b, an oil-resistant layer 1c, and a mold lubricant layer 1d , these layers being successively layered on the hollowed roll 1a in this order. A heating source 1e, such as a halogen lamp, is located within the fixing roll. The pressure roll 2 is composed of a core roll 2a made of metal of high thermal conductivity, and a mold lubricant layer 2d layered on the core roll. The mold lubricant layer 2d is made of polytetrafluoroethylene, for example. A heating source 2e as a halogen lamp, for example, is located within the pressure roll. In FIG. 3, reference numeral 5 designates an oil supply unit for supplying oil, for example, silicone oil, which is used for preventing part of a toner image 4 not yet fixed from being transferred onto the fixing roll 1, viz., an offset phenomenon. Reference numeral 6 designates a cleaning unit, and numeral 7 designates a peeling pawl for peeling the recording sheet 3 from the fixing roll 1 and the pressure roll 2. Reference numeral 8 designates a temperature sensor for sensing temperature on the fixing roll 1 and the pressure roll 2.
To increase the processing speed of the fixing unit, it is necessary to increase the width of the nip area, or the nip width, according to a fixing speed, because the fixing of the toner image is carried out with the cooperation of pressure and thermal energy applied thereto when the recording sheet bearing the toner image thereon passes the nip area between the fixing roll 1 and the pressure roll 2.
The possible methods to increase the nip width are to increase the roll-to-roll load between the rolls 1 and 2, to increase the thickness of the elastic layer 1b of the fixing roll 1, and to increase the diameters of the rolls 1 and 2 . The roll-to-roll load increasing method and the layer thickness increasing method are disadvantageous in the following points. Flexure of the rolls renders the nip width of the nip area nonuniform in the axial direction of the rolls. Further, the fixing operation is irregular or the recording sheet is wrinkled. For those reasons, there is a limit in increasing the roll-to-roll load and the thickness of the elastic layer 1b. Eventually, those methods fail to attain a desired increase of the fixing speed. The roll-diameter increasing method results in increase of the apparatus size, and consequently an elongation of warm-up time taken for the temperature of the rolls 1 and 2 to raise from room temperature to a fusing temperature.
Another fixing method that can gain a great nip width of the nip area is a called roll-belt method which uses a fixing roll with a heating source and an endless belt turning in pressure contact with the fixing roll, the belt being made of polytetrafluoroethylene or silicone rubber (used in place of the pressure roll).
In the roll-belt method, when the endless belt coated with polytetrafluoroethylene is used, the toner, which has been offset to the fixing roll, hardly smudges the recording sheet because it is hardly transferred to the endless belt. However, the recording sheet tends to slip thereon since the surface of the belt has a small friction coefficient. When it slips, the rotating speed of the fixing roll becomes different from that of the endless belt. The toner image, when fixed, becomes offset, viz., an image offset occurs. To prevent this, the fixing roll and the endless belt must be rotated at the same speed. The drivers capable of rotating the roll and belt at the same speed are large and complicated. In the case of the endless belt made of silicone rubber, the surface of the belt has a large friction coefficient. Accordingly, it is free from the image offset problem. However, the silicone rubber belt has another problem that the belt is swelled by an offset preventing liquid, or the silicone oil, in which the belt is immersed. The swelled rubber becomes weak and changes in quality. Further, when the support rolls and the fixing roll are reduced in diameter for the purpose of size reduction, the endless belt is more repeatedly bent or extended with a large curvature. This leads to crack of the belt surface and hence shortening of the lifetime of the belt.